Multi-Layer Reservoir Mapping in Low Resistivity, Low Contrast Clastic Reservoir Using Integration of Ultra-Deep Azimuthal Resistivity 1D and 3D Inversions

Abstract The use of modern ultra-deep azimuthal resistivity (UDAR) tools along with other Logging While Drilling (LWD) sensors increases horizontal well placement efficiency, improving net-to-gross and maximizing reservoir contact. The depth of investigation (DOI) of electro-magnetic (EM) resistivity-based technologies depends on multiple factors: frequency of the EM wave transmitted by the tool, spacing between the transmitter and the receiver antenna, conductivity of the surrounding formation and contrasts between layers. Although the first two factors are relatively controllable, the performance of EM resistivity tools in very low resistivity formation has always remained a challenge, limiting the signal propagation and hence the DOI. This paper presents a novel workflow for utilizing EM resistivity-based reservoir mapping logging while drilling (LWD) technologies for successful well placement and multi-layer mapping in very low resistivity, low contrast and thinly laminated clastic reservoirs. The major challenge for mapping low resistivity low contrast reservoirs is the reduced investigation of the EM wave propagation combined with the weak signal response from the surrounding layers. Resolving multiple layers around the well trajectory then becomes difficult. Pre-well modeling has been performed prior to the start of operations using data from existing offset wells to identify the best configurations and basic settings (spacings, frequencies) for confident reservoir mapping in real time. A novel near-bit UDAR transmitter design was utilized along with a new optimized inversion algorithm was introduced to allow 1D inversion models to map the multiple layers all around the well. This has enabled critical real-time decision making while drilling to steer the well based on petrophysical properties, thicknesses, and structural dip of the target layers. This customized setup was designed specifically for geo-steering and geo-mapping in an extremely low resistivity low contrast environment that is not favorable for the EM resistivity tools. The new workflow has been utilized in multiple wells with low resistivity and low contrast to adjacent beds. The selected tool configuration design optimized the signal to noise ratio (SNR). As a result, and despite the very low resistivity, both 1D and 3D inversions mapped boundaries more than 20ft away, delineating layers that are less than 0.5 Ohm-m. In places, the technology mapped up to 7 thin layers at a distance. Although structural displacements have been encountered while drilling, horizontal wells were successfully placed in the target layers providing maximum reservoir exposure and great improvements in effective net-to-gross (NTG). This paper demonstrates a novel tool setup and workflow to expand the application of UDAR tools for well placement and reservoir mapping in very low resistivity low contrast environments.